Crystal Structure of the High-Pressure Phase of Ca(BH₄)₂ and Further Structural Changes up to 70 GPa

High-pressure (HP) structural changes in calcium borohydride, Ca(BH₄)₂, were investigated up to 70 GPa using X-ray diffraction (XRD) measurements and Raman scattering spectroscopy. At 2.1 GPa, the ambient pressure (AP) phase (α-Ca(BH₄)₂) underwent a pressure-induced phase transition to an HP phase (HP1). Rietveld refinement of the XRD patterns and density functional theory calculations revealed that the crystal structure of HP1 adopts a P2₁/c structure. This structure is close to that of the AP phase, with a volume change of only about 1.7%. Upon further compression of HP1, another pressure-induced phase transition was suggested above approximately 20 GPa; however, the structure of this new HP state (HP2) could not be determined. Raman scattering spectra also revealed the two pressure-induced phase transitions, which are consistent with the XRD results. Although Raman measurements demonstrated that the short-range structural feature was preserved in HP2, long-range ordering was not observed upon XRD measurements, suggesting that HP2 may be an amorphous-like disordered state. Among these phase transitions, the transition from the AP phase to HP1 was reversible, whereas HP2 did not revert to HP1 or the AP phase upon decompression. The sample recovered from HP2 exhibited XRD patterns and Raman spectra similar to those of γ-Ca(BH₄)₂.